Measuring radio signals without dB would be impractical. Decibels make life simple for radio listeners.
Decibels or dB are logarithmic ratios that help us deal with really large or small numbers. Named after Alexander Graham Bell, decibels have no units. They are ratios, not measurements, and must be referenced to some specific unit such as a volt or watt.
You will discover a strong relationship between decibels and the behavior of electric fields. Here’s the reason. As radio signals travel through space, their strength declines according to the “inverse square law”, or 1/(distance)². The decline is exponential, not linear. Decibels provide a shorthand way of counting massive changes in power or amplitude.
Say I transmit a 100 watt signal from Calgary to Toronto. By the time it arrives there, the power of my signal has declined to 20 picowatts. In case you are math challenged, that’s a power drop ratio of five trillion. But don’t worry. My friend in Toronto will still hear me with an S8 signal, which means “strong”.
Decibels Make Life Simple – Measuring Up
Radio listeners use three different scales: S-units, dBm, and dBμV. Here’s what they mean.
dBm is the power of a signal referenced to 1 milliwatt. A power measurement is always referenced to an impedance: 50 ohms for radio, or 600 ohms for audio or telephony. One watt is +30 dBm. My 50 picowatt signal in Toronto is -77 dBm. Right now, you cell phone needs a -90 dBm signal to complete a call, your WiFi needs -70 dBm for broadband coverage, and your GPS works with really small signals down at -130 dBm.
dBμV is the field strength or voltage of a radio signal referenced to 1 microvolt, again across a 50 ohm load. Your local AM or FM broadcasters provide coverage with 74 dBμV (or five millvolts). By contrast, a really weak shortwave signal is about 0.1 μV or -20dBμV.
There is a great conversion calculator to help you learn and use this stuff at Cantwell Engineering.
Lastly, let’s not forget the trusty old S-meter. If properly calibrated, the S9 reading (very good strength) measures -73 dBm or 50 μV. Each S-unit below S9 represents a drop of signal voltage by half, or 6 dB. You will find that S0 represents -127 dBm, which is pretty much the weakest signal you can hear. On the other hand, your local broadcaster is S9+40,
Most radio operators use S-meters, but these are giving way to dB measurements on software defined radios. And for good reason, because S-meters are rarely well calibrated. We will see why later.